1. Field of the Invention
The present invention relates to a variable-capacitance device and a semiconductor integrated circuit device having such a variable-capacitance device.
2. Description of the Related Art
Heretofore, many variable-capacitance devices formed on semiconductor substrates are in the form of a variable-capacitance diode whose capacitance can be controlled by an impressed voltage using the diffusion capacitance of a pn junction. In a variable-capacitance diode having a diffusion layer of a hyper abrupt junction whose impurity concentration is reduced perpendicularly from the surface of a pn junction, for example, the capacitance of the pn junction uniformly varies exponentially as the impressed voltage varies. More specifically, as shown in FIG. 1(A) of the accompanying drawings, a conventional variable-capacitance diode comprises a p-type diffusion layer 1 of high impurity concentration, an n-type diffusion layer 2 forming a hyper abrupt junction with the p-type diffusion layer 1, and a buried n.sup.+ region 3, each of the layers 1, 2 and the region 3 having an impurity concentration profile as shown. The capacitance C of the variable-capacitance diode shown in FIG. 1(A) uniformly varies exponentially as the impressed voltage V varies as shown in FIG. 1(B) of the accompanying drawings. In FIG. 1(B), the abscissa and ordinate indicate logarithmic representations. The capacitance C of the variable-capacitance diode and the impressed voltage V satisfies the following relationship: EQU C=C.sub.D .multidot.(V.sub.D -V).sup.-n ( 1)
where C.sub.D is a proportionality constant determined by the impurity concentration profile, and V.sub.D is the diffusion potential. Since the exponent "n" in the equation (1) can have a large value of "1" or "2" with the hyper abrupt junction, the capacitance C varies at a large rate of change as the impressed voltage V varies.
FIG. 2(A) is a cross-sectional view of another conventional variable-capacitance device.
The conventional variable-capacitance device, generally denoted at 500 in FIG. 2(A), for example, as described in Japanese Patent Specification No. 245282/85 has a capacitance which varies stepwise as the impressed voltage varies as shown in FIG. 2(B). The variable-capacitance device 500 comprises a semi-insulating substrate 501, an operating layer 502 formed in the semi-insulating substrate 501 and having a region whose deepness varies stepwise from the surface thereof underneath an anode electrode 505, an insulating film 503 formed on the surface of the semi-insulating substrate 501, a cathode electrode 504 formed on the insulating film 503 and extending through an opening defined in the insulating film 503 in electric connection to a deeper region (shown on a left-hand side) of the operating layer 502, and the anode electrode 505 which is formed on the insulating film 503 and extends through an opening defined in the insulating film 503 in electric connection to the region (shown on a right-hand side) of the operating layer 502 whose deepness varies stepwise.
When an anode voltage V.sub.A is applied to the anode electrode 505, the depletion layer spreads vertically in the direction of depth from the anode electrode 505. The capacitance C is determined by the depth of the depletion layer and the spread area thereof. Therefore, when the depletion layer spreads to the left further from the region of the operating layer 502 whose deepness varies stepwise, the capacitance C is abruptly reduced because the effective spread of the depletion layer is reduced in the stepped region. Consequently, when the anode voltage V.sub.A is increased, since the capacitance C is quickly reduced as the depletion layer spreads to the left further from the region of the operating layer 502 whose deepness varies stepwise, the capacitance C of the variable-capacitance device 500 varies stepwise as the anode voltage V.sub.A varies as shown in FIG. 2(B). In FIG. 2(B), the abscissa and ordinate indicate logarithmic representations.
In the above variable-capacitance diode with the diffusion layer of a hyper abrupt junction whose impurity concentration is reduced perpendicularly from the surface of the pn junction as shown in FIG. 1(A), the capacitance C varies at a certain ratio as the anode voltage V.sub.A varies as shown in FIG. 1(B). Therefore, the variable-capacitance diode can have a desired capacitance based on the anode voltage V.sub.A. One problem, however, is that when the anode voltage V.sub.A varies, the capacitance C shifts from a desired capacitance.
On the other hand, the variable-capacitance device 500 shown in FIG. 2(A) is disadvantageous in that a plurality of selective ion implantation steps using different mask patterns are required to form the operating layer 502 whose deepness varies stepwise, and hence the fabrication process is complex. The variable-capacitance device 500 cannot practically employ a silicon substrate because if the substrate in which the operating layer 502 is formed were not the semi-insulating substrate 501, the depletion layer would spread downwardly of the operating layer 502 and an avalanche breakdown would occur. Furthermore, inasmuch as the deepness of the operating layer 502 differs stepwise, sufficient efforts have not been made to reduce a change in the capacitance brought about by a change in the anode voltage V.sub.A in regions where the capacitance is relatively flat in FIG. 2(B), though it is possible to vary the capacitance stepwise as the anode voltage V.sub.A varies.